Crude oils and middle distillates recovered through the distillation of crude oils, such as gas oil, diesel oil or fuel oil contain, depending on the origin of the crude oils, different amounts of dissolved long-chain paraffins (waxes). At low temperatures, these paraffins crystallize out as flake-like crystals, partly with oil inclusions. This considerably impairs the flowability of the crude oils and the distillates recovered from them. Solid deposits occur which often lead to breakdown during the recovery, transportation and use of mineral oil products. For example, in the cold season, the filters of diesel engines, furnaces, and boiler plants often become clogged, thus preventing reliable dosing of the fuel and ultimately resulting in complete breakdown of the fuel supply. The transportation of middle distillates over large distances in pipes can also be impaired in winter due to the paraffin crystals separating out.
It is known that undesired crystal growth can be prevented by suitable additives. Such additives, which are known under the terms pour-point depressants and flow improvers, change the size and shape of the wax crystals and thus counteract the increase in viscosity of the oils.
The flow and cold behavior of mineral oils and mineral oil distillates is indicated by the pour point (determined according to DIN 51597) and the cold filter plugging point (CFPP, determined according to DIN 51428). Both values are measured in .degree.C.
Typical flow improvers for crude oil and middle distillates are copolymers of ethylene with carboxylic acid esters of vinyl alcohol. Thus, according to DE 11 47 799 B1, oil-soluble copolymers of ethylene and vinyl acetate having a molecular weight of between about 1,000 and 3,000 are added to crude oil distillate road and heating fuels with a boiling range of about 120.degree. to 400.degree. C. Copolymers containing about 60 to 99 weight percent of ethylene and about 1 to 40 weight percent of vinyl acetate have proved particularly suitable.
Furthermore, it is known that those of the above copolymer compositions which are particularly effective have been prepared by radical polymerization in an inert solvent at temperatures of about 70.degree. to 130.degree. C. and pressures of 35 to 2,100 atmospheres above atmospheric pressure (DE 19 14 756 B2).
The subject matter of the DE 21 02 469 C2 is the use of ethylene copolymers having a molecular weight of 500 to 4,500 and a comonomer content of 30 to 75 weight percent. They are obtained by joint polymerization of ethylene and vinyl esters having 4 to 10 carbon atoms, together with acrylic acid and methacrylic acid esters derived from alcohols having 1 to 12 carbon atoms, in the absence of solvents and emulsifiers.
The effectiveness of the known additives in improving the properties of mineral oil fractions depends on the origin of the mineral oil from which they were obtained and thus on their composition. Prior art additives, which are admirably suitable for adjusting certain characteristic values of fractions of one crude oil, can produce completely unsatisfactory results with distillates of crude oils of a different origin.
For economic reasons, there is interest in increasing the yield of middle distillate road and heating fuels. One way of achieving this is to add higher percentages of heavy gas oil to the middle distillate blends. Another possibility of improving the middle distillate yield consists in increasing the boiling interval, i.e. separating middle distillate fractions which cease boiling above 370.degree. C. Both processes lead to an increase in the higher molecular weight n-paraffin percentage in the middle distillate road or heating fuel. The known additives, such as copolymers of ethylene and vinyl acetate, lead to an unsatisfactory improvement in the flowability of middle distillate fractions which cease boiling above 370.degree. C.